1. Technical Field
The embodiments herein generally relate Frequency Modulation (FM) radio receivers, and, more particularly, to a FM radio receiver with pilot based adaptation.
2. Description of the Related Art
A radio receiver is an electronic circuit that receives its input from an antenna, uses electronic filters to separate a wanted radio signal from all other signals picked up by this antenna, amplifies it to a level suitable for further processing, and finally converts the signal into a form usable for the consumer (e.g., sound) through demodulation and decoding. Frequency Modulation (FM) conveys information over a carrier wave by varying its frequency, unlike amplitude modulation, in which the amplitude of the carrier is varied while its frequency remains constant. In analog applications, the instantaneous frequency of the carrier is directly proportional to the instantaneous value of the input signal.
This form of modulation is commonly used in the FM broadcast band. It is important that stereo broadcasts should be compatible with mono receivers. For this reason, the left (L) and right (R) channels are algebraically encoded into sum (L+R) and difference (L−R) signals. A mono receiver will use just the L+R signal so the listener will hear both channels in the single loudspeaker. A stereo receiver will add the difference signal to the sum signal to recover the left channel, and subtract the difference signal from the sum to recover the right channel. Stereo FM signals are more susceptible to noise and multi path distortion than are mono FM signals. In addition, for a given RF level at the receiver, the signal-to-noise ratio for the stereo signal will generally be worse than for the mono receiver.
FIG. 1A illustrates a decoding scheme used in a traditional FM receiver with only mono reception. A FM signal 100 first passes through a FM decoder 102, which converts the FM signal 100 to a composite audio signal. Then, the composite signal is sent to an audio low pass filter (LPF) 104. The output of the audio LPF 104 is a mono L+R audio. FIG. 1B illustrates a decoding scheme used in a traditional FM receiver with only stereo reception. The FM signal 100 first goes through the FM decoder 102, which converts the FM signal 100 to a composite audio signal. Then, the composite signal is sent to a stereo decoder 106 to generate separate coarse audios for left and right ears (L, R).
Finally, the coarse L and R audios are sent to the Audio LPF 104 to get the final L and R audios or the stereo audio. FIG. 1C illustrates a simple FM adaptation scheme between mono and stereo used in a traditional FM receiver. In the FM adaptation scheme of FIG. 1C, the composite signal output from FM decoder 102 is passed through a pilot detector 108. The pilot detector 108 detects whether the pilot signal for stereo decoding is present. The outputs from the stereo decoder 106 and the pilot detector 108 are passed through a multiplexer 110 and then through the audio LPF 104. If the pilot signal is not present, only a mono reception (L+R) is performed.
If the pilot signal is present, then only a stereo reception (L, R) is performed. Thus, traditional FM radio receivers perform stereo decoding or mono decoding using the same low-pass filter for a final audio, regardless of the received signal quality. Some traditional FM radio receivers perform a simple adaptation based on the presence of pilots in the decoded signal. If the pilot is present, they perform stereo decoding while if it is not present, they perform mono decoding. Thus, there is a need for an improved pilot based adaptation technique for a FM radio receiver.